Influenza Linked to Absolute Humidity

Feb. 13, 2009 -- Absolute humidity more than relative humidity is linked to
influenza and the spread of the virus, a new study suggests.

Absolute humidity is a measure of the actual amount of water in the air,
regardless of temperature. Relative humidity is the ratio of air water vapor
content to the saturating level, which varies with temperature.

Scientists have long suspected a link between humidity and flu transmission
and prevalence, but the main suspect has been relative humidity, says lead
study author Jeffrey Shaman, PhD, of Oregon State University, who specializes
in studying ties between disease transmission and climate. But the real
culprit, he contends, is absolute humidity. The researchers explain that when
humidity is low, as in peak flu months of January and February, the virus
appears to thrive longer, and transmission rates zoom.

Shaman took a fresh look at data from a 2007 study that had found a “tenuous
relationship” between flu transmission and relative humidity. He reused that
research data, but substituted absolute humidity for relative humidity to
search for correlations.

That led to additional investigation of the relationship between absolute
humidity and survival of flu virus, or the length of time the bug remains
viable once airborne.

That 2007 study, in general, found more infections when it was colder and
drier.

But Shaman and co-author Melvin Kohn, an epidemiologist with the Oregon
Department of Health Services, found that relative humidity could explain only
about 12% of the variability of influenza virus transmission.

They showed that relative humidity explains only about 3% of flu virus
survival. After looking at the data focusing on absolute humidity, they found
“dramatic” rises in both variability of transmission (from the 12% found
earlier, to 50%) and survival (from 36% to 90%).

So conventional wisdom, backed by much research pointing at relative
humidity, may be all wet, the researchers suggest, adding that absolute
humidity is the more likely villain.

The conclusion, they say, is clear: “Outbreaks of influenza typically occur
in winter when low absolute humidity conditions strongly favor influenza
survival and transmission.” They add that "these findings also suggest that
humidification of indoor air, particularly in places where transmission to
those at high risk for complications, such as nursing homes and emergency
rooms, may help decrease the spread and toll of influenza during influenza
season."

The study is published in the Proceedings of the National Academy of
Sciences.